My back labour felt like I tore a muscle in my back, and it sent shooting pains down my legs (mostly during active labour). While my labour was just begining (like contrax 15 or more minutes spaced) it felt like someone was poking me in the back with a sharp stick with the contractions, and just a little achey in between..
I would call your ob, and if its not labour, then maybe see a chiropractor, could be your back needs adjusting